Pop quiz time! You're wearing plate leggings that have 1104 armor, 34 Str, 57 Sta, and 38 Haste Rating. You complete a quest and one of the reward options is a different set of plate leggings with the same item level, also 1104 armor, but this one has 22 Str, 45 Sta, and 53 Critical Strike Rating. Which set of pants should you wear for solo DPS? Will the answer still be the same when you gain your next level? How about if you respec?
In fairness, this situation is not unique to WoW's newest expansion. Combat Ratings were introduced in TBC as a way to keep players from amassing 100% crit, etc. Things have gotten more complicated in Wrath of the Lich King for several reasons.
Ratings are everywhere
The change to combat ratings has allowed much more gear to have these stats than previously; if level 60 gear was going to offer a crit bonus, it was 1%, 2%, or nothing. As a result, it was easier to get away with simply not caring what the ratings were worth until level 70, at which point you could memorize the 3-4 ratings that are relevant to your class. Ratings allow Blizzard to dole out small amounts of multiple stats on the same item, making it even harder to compare than my initial example.
There are more distinct ratings
The later tiers of TBC raiding (ZA and the Black Temple) awarded haste rating and armor penetration, two stats which were not widely used in prior content. These stats are now available on level 70 quest rewards in Wrath, where they join hit rating, crit rating, Attack Power (which can also be gained from pure stats, and is further split into Ranged and Non-Ranged flavors), and Expertise. (That's just the melee DPS stats, there are separate stats for spellcasting and defense.) It's one thing to remember that hit is best until you reach the hit cap (which, incidentally, is not documented in the game UI anywhere), and then you want crit or pure damage, but it's another to try and figure out how armor penetration (which is also going to be converted into a rating), haste rating, crit rating, and attack power compare to one another.
More talents mean more differences between specs
As the talent trees get taller and taller, the differences between two specs of the same classes get greater and greater. Maybe this spec now has talents that are especially dependent on getting critical hits to proc, while this other spec is plagued by slow attack/casting time and needs haste rating.
Ratings Degrade as you Level
If your gear has 50 attack power, it will still have 50 attack power next level, 5 levels, and 10 levels from now. By contrast, the ratings on your gear give your character less benefit per point every level you gain. The author of Rating Buster (a mod I view as nigh essential, which does the math for you on what percentage your ratings are worth at any given level) reports that ratings at level 80 will actually be worth as little as 50% of what they were at level 70. The result is that your non-rating based gear will age more gracefully as you level.
Ratings are worth comparatively less as you get more of them
I'm not entirely clear on all the theorycraft here myself (and I do love my theorycraft), but my limited understanding is that the way the attack table works in WoW means that 1% crit is worth more when you're starting from zero than it is when you're starting from 20%. I'm going to stop talking now before I say something wrong, but the point is that you can't even memorize one number and count on it to still be correct after you've been getting enough gear upgrades (or "downgrades" as a result of your existing rating decaying).
Gear upgrades come faster while leveling than at the cap
This is a temporary problem, but one I'm experiencing now. Once you've been at the cap for a while, you have time to get to know what kind of gear you're wearing and where your next potential upgrade is coming from. While leveling, upgrades come more frequently. You probably haven't taken the time to plan out your choices on every single minor quest reward, and you may not even know the current rating conversions for your level. Again, this is temporary, but it only serves to add to the confusion.
What can be done?
Well, the band-aid fix is to download Rating Buster and EQCompare (for reasons beyond my comprehension, Blizzard's default UI will not compare gear tooltips for items in your bags to what you're currently wearing, even though the UI does offer this option at vendors, quest reward windows, and the Auction House) and keep a spreadsheet on hand to crunch the numbers for your current level. You can also download a theorycrafting mod (such as Dr. Damage) that evaluates your spec and gear and determines what, for example, the average damage on your Frostbolt is going to be so you can compare it before and after equipping a new item. The numbers behind these mods are going to be debatable at times though (see previous discussion RE: crit procs vs speedier attacks), and need to be reverse engineered manually every patch. (Blizzard could, I suppose, cut out the middle man and add their own version of this to the default UI, but that's how the UI slowly gets more and more crowded over time.)
At the end of the day, though, this can only be changed at the design level. Blizzard apparently feels that the choice between various highly specialized ratings is worth the added complexity (but that actually making the numbers transparent so you'd stop stacking hit rating when you're capped would be too complicated). And maybe they're right. Ask again in the comments next year when I'm tearing my hair out trying to determine whether whupass rating is superior to coolness factor and boredom penetration.